Dirca by Day

Since starting my M.S. degree in horticulture at Iowa State last fall, I’ve wanted to write on numerous occasions about my research, but just haven’t found the time.

Though I moonlight in my professional life as a plant breeder, by day I’m a plant ecophysiologist, someone who studies and describes physiological mechanisms underlying ecological observations and questions (i.e. why do plants respond to their environment the way they do). Specifically, I study the effects of provenance (the biogeographical origins of plants) and the implications those effects have on the performance of plants in the landscape. Provenance is a big deal, though sadly often taken for granted by professionals and novices alike. Particularly for plant species with broad distributions, provenance plays a huge role in how germplasm collected in the wild ultimately performs horticulturally. Many plant species in horticultural commerce today stem from a single or just a few collections of that species in the wild. The genetic diversity (and thus capacity for those species to exhibit a multitude of traits and responses to environmental factors) is often greatly diminished in landscapes. I’m often humored when I read treatises on various genera at how matter-of-fact authors are in their appraisals of species, as if to suggest that the few horticultural forms of a given taxa represents the majority. In fact often quite the opposite is true. As horticulturists, we have no doubt dismissed more than one new species collected from near or afar, simply because we brought into commerce a lackluster form.

To study provenance, I spend my days investigating the world of the genus Dirca. The genus is manageably small, with only three species in wide acceptance.

Dirca palustris (eastern leatherwood) occurs in highly localized populations across the eastern one-third of the United States from Maine and Ontario, west to North Dakota, south to Oklahoma and Louisiana, and east to northern Florida.

Dirca mexicana (Mexican leatherwood) is known from a single reported population in Tamaulipas, Mexico, and has not been introduced to horticultural commerce.

Dirca occidentalis (western leatherwood) occurs in six counties near the San Francisco Bay in California.

All three sport dangling yellow flowers in early spring, with D. occidentalis having the largest flowers of the genus.

A fourth species recently described as Dirca decipiens (decipiens translates from Latin to mean deceiving) is known from a few disjunct locations in northwestern Arkansas and one populous outcropping within the confines of the Overland Park Arboretum outside of Overland Park, Kansas. Little is known about this new species, though it is morphologically intermediate between D. palustris and D. mexicana. Details of the discussion about “the deceiver” would seem a little out of place here, more the substance of conversation between a geeky graduate student and his colleagues than a plantsman and his friends. Horticulturally speaking there isn’t much if any debate–it’s virtually unanimous that the genus remains woefully underappreciated.

The genus lends itself to inquiries of provenance, chiefly for two reasons. First, the most widespread species, Dirca palustris, occurs in highly localized populations across a vast geographic area (see range above). Across that range, a number of horticulturally characterizable provenances exist–the comparison and study of which constitute the majority of my research. Second, the other congeners (members of the genus) exist in highly localized, endemic populations, limited to geography on a much finer scale. This ability to compare between two different kinds of ranges facilitates a variety of questions into how we might grow these species horticulturally. Other colleagues of mine study the evolutionary history of the genus and its family members, a completely fascinating arena that generates about as many questions as it answers (perfect!)

In summary, Dirca are fascinating plants, if not a little nerdy. Hailing from the daphne family (Thymelaeaceae), leatherwoods thrive in shade, often growing near moving water though rarely if ever in it. As arborescent (tree-like) shrubs, they harbinger the earliest signs of spring, blooming before most trees and shrubs, and epitomizing those famous words by Gertrude Wister– “The flowers of late winter and early spring occupy places in our hearts well out of proportion to their size.”

4 Responsesso far.

Kelly, I sent some Dirca decipiens seed to Dr. Graves at Iowa State in May of 2010. I am a long time volunteer at the Overland Park Arboretum and I gathered the seed at the Arboretum. I understand the seed were for planting and I want to know if any of them came up.

May 24, 2010 I also planted about 500 Dirca decipiens seed in an outdoor seed bed, under shade tree mix of Walnut, Hackberry, Locust and Coffeetree. The seed bed had about 2″ of sphagnum peat moss tilled into the top 6″ of soil. Germination on the seed was excellent and I have since transplanted about 300 of the seedlings to small 5″ deep cell pots. Do you know if the seed I sent to Dr. Graves germinated and if so how did you handle them.

I would appreciate a reply when you have time. Best regards, Ken O’Dell 11485 W. 303rd,. St. Paola KS 66071 913 837-5112

[…] mutual love for eastern leatherwood or Dirca palustris. After spending much of the last two years studying this species and its congeners, I couldn’t help but celebrate the completion of my M.S. degree this year by including this […]